The invention relates to a process for fusing toner on a carrier or on a printing material, particularly on a sheet-type or a tape-type printing material, preferably for a digital printer.
Furthermore, the invention relates to a device for heating printing material and/or toner, particularly to fuse toner on a carrier or printing material, particularly a sheet-type or tape-type, preferably for a digital printer, preferably for performing the above-mentioned process.
With digital printing, particularly electrostatic or electrophotographic printing, a latent electrostatic image is produced, which is developed by the loaded toner particles, which, on their part, are transferred onto a printing material receiving the image, e.g. paper. There the image transferred to the printing material is fused by heating and softening of the toner and/or heating of the printing material. By, and during, this process the toner particles are fused on the printing material and, if necessary, also on each other.
The use of microwaves is known for the fusing of toner on the printing material. Since the absorption of microwave energy in the toner is usually at least one order of magnitude smaller than that in the printing material, the printing material is preferably heated by the microwaves and the printing material heats on its behalf the toner found thereupon, and indeed up to a temperature that fuses the toner on the printing material. It is well known that, with the use of microwaves for fusing the toner, characteristic values of the printing material used, such as weight, moisture and composition are critical and must be taken into consideration.
Thus, for example, an image-fusing device is known from U.S. Pat. No. 4,511,778, which fuses a toner image on a printing material, particularly a sheet of paper, by high frequency waves, particularly microwaves. One aspect of the known device is the possibility of producing the microwaves as a function of the size of the printing material, so that, by taking this size into consideration as a characteristic value of the printing material, an appropriate melting and fusing of the toner is ensured. This is a process that is quite general and only takes into consideration an immediately obvious size of the printing material, which is determined prior to the fusing thereof for the operation of the device, possibly according to a consideration that a larger piece to be heated, due to its greater heating capacity, requires all in all more energy than a smaller piece to be heated.
Due to this general guideline, however, other critical aspects for the use of microwaves for fusing the toner are ignored. Thus, for example, the cited process is only applicable to black and white printing with a paper weight within only a small range of widths, while the possibly different behavior of a multicolor toner with different paper weights, which may also have different water contents, are not taken into consideration in this general process, which is determined by the size of the printing material. With color printing, the toner may, for example, have four different toner layers. Here, the maximum thickness of each toner layer on the image-carrying substrate or printing material is 100%, whereby a maximum total weight of the toner layers in the toner image is 400%. The thickness of a single color customarily lies in the range of 0% to 100% thickness, and a color toner image is in the range of 0% to 290%.
Furthermore, with the use of sheet-type printing material, the problem may arise that the edge of the sheet that has been irradiated with microwaves may be energetically processed in a different manner than the middle section of the sheet. This may result in a printed product that is uneven.
In addition, the fusing of conventional toner using only microwaves under such conditions only results in an incomplete melting of the toner, depending on its layer thickness, or it results in heating with blistering in areas of the toner. In addition, the fusing of the toner on the printing material is insufficient under the circumstances, because, for example, the fusing of the toner on the printing material is insufficient due to the fact that viscosity of the melted toner is too high. Problems can occur then, especially if a printing material is printed in two successive printing processes on both sides.
Due to the possible occurrence of the problems described, the use of microwave irradiation is usually not trusted for fusing the toner, but the toner is in practice heated without microwave irradiation and is fused on the printing material with a pair of heated, pressurized rollers. However, contact-free fusing is principally desired for preservation of the printed image. Other advantages of the contact-free fusing are the prevention of adhesive wear and tear and the increased service life of the device used as well as ensuring a better reliability of the device.
The desire of this invention is to make adequate fusing of the toner on a printing material, or its preparation, using microwaves possible, preferably also for multicolor printing on sheet-type printing material and preferably with adjustment to the prevailing special conditions.
With respect to the process, it is provided according to the invention, that the printing material with toner is irradiated with microwaves from at least one microwave conductor, which is heated to melt the toner and that a toner is used that shows a quick transition from its solid state to its liquid state when heated.
In this process according to the invention, a dry toner can be used, for example, that is still quite solid at a medium temperature of some 50xc2x0 C. to 70xc2x0 C., so that, by a conventional process, it can be ground to a desired medium toner size of 8-4 micrometers. For example, said dry toner is not yet sticky and does not melt at development temperatures, but already flows very easily and has a lower viscosity at higher temperatures of some 90xc2x0 C. Thus, if used with capillaries, the dry toner can be fused and adheres without outside pressure and contact-free on the printing material, and when cooled, it becomes hard again and is fused. It has indeed a good surface luster suitable for the printing material, and, in particular, without developing grain boundaries. The latter already also plays a significant roll in the color saturation with color toner.
With this process, in connection with the toner according to the invention, the ratio of the values of the elastic modulus Gxe2x80x2 at the reference temperature value can be calculated from the initial temperature at the beginning of the glass transition of the toner plus 50xc2x0 C. to the value of the elastic modulus at the initial temperature itself  less than 1E-5, preferably even  less than 1E-7, whereby E represents the exponent on base 10. The initial temperature at the beginning of the glass transition of the toner is preferably determined as that temperature value at which the tangents to the functional behavior of the elastic modulus Gxe2x80x2 intersect as a function of the temperature before and after the glass transition. The glass transition of the toner from its hard state to its liquid state preferably takes place in a temperature interval or temperature window of approximately 30xc2x0 C. to 50xc2x0 C. order of magnitude. This range should lie above 60xc2x0 C., preferably between approximately 70xc2x0 C. to 130xc2x0 C., and most preferably between 75xc2x0 C. and 125xc2x0 C.
Another embodiment of the process according to the invention is distinguished in its adaptation to special ratios, in that at least one physical process parameter is controlled and/or regulated as a function of a parameter correlating to the energy charge in the printing material with toner. According to this aspect of the invention, a simple general guideline is thus not envisaged, but advantageously a control regulated to the actual, preferably measured ratio. Here the abovementioned energy charge basically corresponds to a microwave power received from the total system of the printing material and toner, so that, according to the invention, corresponding to the actual ratios, the power transmitted is compared and regulated with the power received. This basically corresponds in turn to an efficiency control and/or regulation. Here it is generally considered, in particular, to have a control on the side of the transmitter in the widest sense, which can also be considered to be a microwave source, and/or on the side of the receiving toner-printing material system, or the use of one.
To this end, the invention recommends that it is preferable to regulate the power of the microwave conductor and/or regulate the movement speed of the printing material and/or to regulate the frequency of the microwaves. The latter measure is preferable to achieve a higher energy absorption directly in the toner itself, and thereby a more precise effect on its fusion than can be achieved indirectly and problematically with the printing material. The invention recommends the temperature of the printing material or the microwave energy reflected from the toner-printing material system, and which is thus unabsorbed, as the measurable parameter for the dependent regulation. Other measurable parameters could be, but are not limited to, the weight/the thickness or the water content of the printing material or the density and luster of the toner layer.
In principle, all frequencies of the microwave range from 100 MHz to 100 GHz can be used. As a rule, the ISM frequency released for industrial, scientific or medical utilization, particularly 2.45 GHz, is used. A use of other frequencies in the known frequency range may, however, advantageously lead to a situation where a greater portion of radiant energy is absorbed than usual from the toner and not only from the printing material. Separate protection is required for a device of the type mentioned, which is distinguished in its independent solution of the task at hand, in that at least one transmitter is envisaged that transmits microwaves, whereby, for the purpose of irradiation and heating, the toner has a quick transition from its hard state to its liquid state when heated. To this end, one or more adjustable operating parameters are preferably envisaged.
Another embodiment of the device according to the invention is distinguished by at least one microwave conductor with a meandering or serpentine course. The structure of the microwave conductor according to this aspect of the invention has the advantage that a relatively large microwave conductor length is made available as the interaction length with the printing material for the continuous wave field for the application of the microwave energy. To this end, the microwave conductor is preferably conducted repeatedly back and forth crosswise to the transport direction over the transport route for the printing material. In this manner, a uniform and homogenous heating of the printing material with high efficiency is achieved.
The meandering shape can be relatively compactly prepared such that the meandering section is densely packed together. In addition, the microwave conductor width for a corresponding increase of the field strength can be reduced. A compact configuration is particularly desirable with a printing of the printing material in the face printing and the reverse printing and the required fusing of the respective toner image for this purpose. Depending on the microwave energy supplied, the maximum field strength should be 3 kV/mm, preferably approximately 0.2 kV/mm to approximately 1.0 kV/mm.
With each passage through a meandering segment, the electric field strength E decreases based on the absorption through the printing material and respectively, the toner image. Since the absorption is not linearly dependent (xcx9cE2) on the electric field strength and is linearly dependent on the respective width of the microwave conductor, the decreasing field strength can be partially compensated by a suitable successive reduction of the width of the successive meandering segments. Preferably, a compensation can take place in addition or instead of by a suitable convergent or conical passage of the width of the microwave conductor. The goal in each case is an extensive constant electric field strength over the length of the microwave conductor and particularly also within the meandering segment. This likewise can be achieved through the abovementioned convergent passage. In addition, these measures reduce the size of the meandering in the transporting or processing direction.
In addition, the decrease of the absorption inside a meandering segment can be partially compensation by the convergent passage. To this end, the convergent geometry is configured in such a way that a smaller electric field strength prevails at the beginning than at the end of the meandering segment. Since the absorption is not linearly dependent (xcx9cE2) on the electric field strength and is linearly dependent on the respective width of the microwave conductor, a suitable ratio of the microwave conductor width and the electric field strength with extensive constant absorption can be found over the length of the meandering segment.
The transport direction of the printing material can occur in both directions perpendicular to the extent of the meandering segments, however, the microwave power must be taken into consideration and adjusted accordingly with the feeding and discharge of the printing material. The number of meandering segments is dependent upon the necessary power absorption and upon the temperature of the printing material and the homogeneity of its heating and drying.
The device according to the invention is not only just suitable fuser, but it could also be advantageously used as a preheating device. It would also be suitable as a conditioning device for conditioning the printing material, particularly paper. A change of the printing material can then readily take place by the application of heat prior to the printing process. The device according to the invention is preferably envisaged for a digital multicolor printer.